This invention relates to a process for the dry carbonation of an alkali metal or ammonium carbonate, utilizing a low carbon dioxide-containing carbonating gas stream. More particularly, the invention relates to such a process for the production of an alkali metal or ammonium bicarbonate sorbent useful in the desulfurization of flue gas.
The conventional technique utilized in the commercial production of sodium bicarbonate is the solution process. In the solution process, soda ash is dissolved in spent reaction liquor from prior reaction, consisting of water and small quantities of dissolved soda ash and sodium bicarbonate. The solution is then carbonated to precipitate crystals of sodium bicarbonate. The sodium bicarbonate crystals are separated from the liquor and dried to yield highly purified, high density crystals. Disadvantages of the conventional method are that the procedure requires several steps, and necessitates the use of separation equipment, drying of the product, and the handling of large volumes of liquids.
It has also been proposed to make sodium bicarbonate by various dry carbonation techniques. In U.S. Pat. Nos. 276,990 Carey et al.) and 574,089 (Hawliczek), a sodium bicarbonate product is formed by placing hydrated soda ash in a revolving cylinder and then introducing carbon dioxide into the cylinder. In both patents, reaction times are of the order of five to six hours.
U.S. Pat. No. 3,647,365 (Saeman) teaches a process in which hollow sodium bicarbonate beads of low density are prepared in a multistage reactor from hydrated soda ash, small amounts of water and carbon dioxide. This process requires several steps and must proceed slowly, with carbonation times exceeding one hour and drying times up to eight hours. The soda ash must first be hydrated in a separate step and the reaction must occur at a temperature above 95.7.degree. F. to produce commercially acceptable reaction rates.
More recently, Krieg et al. U.S. Pat. No. 4,459,272, owned by the assignee of the present invention, describes a process for the preparation of sodium bicarbonate by the reaction of a solid, particulate sodium carbonate-containing material with liquid water in a carbon dioxide-rich atmosphere. In the Krieg process the particulate mass is mixed in an internally agitated or externally rotated or vibrated reactor with the water and carbon dioxide. The reaction is carried out at temperatures of from 125.degree. F. to 240.degree. F. under atmospheres containing from 20% to 90% carbon dioxide by volume. The process is carried out under reduced water vapor partial pressures to promote evaporation of water from the surfaces of the reacting carbonate particles, and to maintain high carbon dioxide partial pressures in the reactor atmosphere. Products formed by the process have apparent bulk densities as high as 50-60 lbs./ft..sup.3.
Each of the previously described dry carbonation techniques is subject to particular disadvantages. In each process, the carbon dioxide concentration must be high and the reaction temperature must also be high, or the reaction rate is prohibitively low. None of these methods can produce sodium bicarbonate at low temperatures and low carbon dioxide concentrations, at commercially acceptable reaction rates.
Sodium bicarbonate has also been produced, as well as utilized, in dry sorbent injection processes for removing sulfur dioxide emissions from the combustion gases of fossil fuel-fired burners. Such techniques have commanded considerable attention recently, particularly since they present the lowest "first cost" alternative for removing potentially dangerous sulfur dioxide from flue gases. Sodium bicarbonate has been demonstrated to be a very effective sorbent in the dry sorbent injection process. However, the cost of pharmaceutical grade sodium bicarbonate, as currently produced, is a major drawback to its use for such purpose.
U.S. Pat. Nos. 3,846,535 (Fonseca) and 4,385,039 (Lowell et al) disclose methods for regenerating sodium bicarbonate from sulfate-containing solid waste formed by dry sorbent injection with sodium bicarbonate. The Fonseca regeneration step is carried out by forming an aqueous solution of the sodium sulfate-containing waste, and treating such solution with ammonium bicarbonate to precipitate sodium bicarbonate. The sodium bicarbonate is then separated, dried and recycled for further use. Lowell et al discloses a regeneration step which involves dissolving the solid desulfurization reaction product in a basic liquor, which contains borate ions and/or ammonia. Carbonation of this liquor results in a sodium bicarbonate precipitate. The Fonseca and Lowell et al processes thus both suffer from the use of complicated and capital-intensive solution operations.
It is among the objects of the present invention to provide an improved process for the production of sodium bicarbonate and other alkali metal or ammonium bicarbonates, which does not require the multiple operations required by prior art solution processes, nor is it limited to use of the high temperature, high carbon dioxide-concentration gas mixtures utilized in previous dry carbonation techniques.
A further object of the invention is to provide such a process which may be readily employed to produce bicarbonate sorbent employed in the desulfurization of flue gases, more efficiently and economically than possible utilizing previously proposed techniques.
These and other objects and advantages of the invention will be described more fully below.